Date of Award

4-2020

Degree Name

Master of Science

Department

Geological and Environmental Sciences

First Advisor

Dr. Matt Reeves

Second Advisor

Dr. Mohammed Sultan

Third Advisor

Dr. Duane Hampton

Keywords

Modeling, nitrogen, transport, groundwater, hydrogeology

Access Setting

Masters Thesis-Open Access

Abstract

Shallow water tables in coastal surficial aquifers limit effective treatment of septic effluent which can result in excess nutrient loading into nearby surface water bodies. Approximately 45,000 septic systems in Charlotte County, Florida transmit effluent into an under studied surficial aquifer and contribute to harmful algal blooms and outbreaks of E. coli. An undeveloped field site was characterized using standard hydrogeologic methods, including a one-year duration natural gradient tracer test, to obtain representative lithology of the sandy surficial aquifer and estimates of groundwater velocity, flow directions, effective porosity and dispersion. These data were used to support the development of a groundwater flow and nitrogen transport model of a nearby coastal subdivision connected to 2000 septic systems with high septic and canal density. Model results were used to assess the impacts of coastal ground water discharge in regions with high septic density near the coastline, and ground water – canal interaction and potential for rapid transport into Charlotte Harbor. Timescales associated with nitrogen removal by natural groundwater flow in the surficial aquifer following instantaneous septic to sewer conversion were on the order of 2-3 years for 50% reduction and 8-10 years for 90% reduction. Canals were found to significantly influence groundwater flow and rapidly convey nitrogen to Charlotte Harbor. Pre and post sewer conversion data on nitrate and total nitrogen in shallow groundwater from a nearby field site was obtained post-model development and supports the timescales predicted by the numerical model.

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